Detonating cords typically contain a secondary high explosive core encased in an outer textile sheath and plastic jacket. Typical explosive materials used are PETN, RDX, HMX, HNS, and PYX. These textile wrapped detonating cords are used extensively in the petroleum exploration and production industry to initiate other explosive components used in various downhole tools. The textile wrapping provides a highly flexible structure that can be easily threaded through perforating guns. Some examples of components that textile detonating cords are used with are perforating shaped charges, setting tools, and similar items. The well locations where these components are used are widely scattered around the world sometimes in very remote locations. It is highly desirable to be able to ship detonating cord by air from a central store location to the remote field location needing the material. However the regulations governing the shipment of explosives by air are quite stringent.
Basically the regulations require that detonating cord explosive materials be packaged in such a manner that an ignition or detonation in one container shall be confined to that container and will not propagate to another container. In practical terms, this means that the maximum amount of detonating cord allowed to detonate in a package is twelve inches to thirty-six inches.
The prior art has several examples of packaging methods that have been used to meet the air shipping regulations for explosive materials. U.S. Pat. No. 4,586,602 discloses a detonating cord transport system where the detonating cord is wound on a plurality of separator support members that provide crossover locations at frequent intervals. At these crossover points, a severing means is wrapped around the cord so that the detonation of one cord portion will sever the continuing cord length at the crossover point without initiating the cord. The maximum length of cord that can detonate without encountering a crossover point is approximately one foot. Packaging of detonating cord using this method is quite laborious and involves inserting severing means around the cord and cable ties to anchor the cord in position.
U.S. Pat. No. 4,817,787 discloses a different packaging method where a mounting board of insulating material, such as expanded polystyrene, is used to hold the cord. Walled paths are molded into the mounting board through which the cord can be threaded. The cord path has a series of loop regions and adjoining parallel regions through which the parallel cord is separated by the wall. The purpose of the wall is to provide a safety distance where the detonation of a length of cord will cause the adjacent parallel length of cord to be severed without initiation. The minimum wall thickness required for the expanded polystyrene is about 0.205″ minimum.
U.S. Pat. No. 4,895,249 discusses a packaging method that is claimed to be an improvement over the detonating cord transport system disclosed in the '602 patent. This patent discloses a method that increases the labor efficiency of packaging detonating cord and efficiencies in the quantity of detonating cord per package. In this patent, the detonating cord is also wound on a plurality of separator support members. The cord is wound in loops that cross over itself at frequent locations. At the crossover points a severing means is inserted which serves as a means of stopping the detonation at the crossover point. A preferred example of a severing means is a nylon-reinforced rubber hose that is slit and placed around one cord section at the crossover point. Each separator support layer can accommodate about 25 feet of detonating cord. Twenty stacked layers will therefore allow a total of 500 feet of detonating cord to be shipped in one package.
The prior art disclosed in both the '602 patent and the '787 patent rely on a separate severing means to actually cut the detonating cord. For this system to work, the detonating cord must follow a path very close to the adjacent strand being actually severed. The detonation of the cord will accelerate the independent severing means at high velocity. The material being accelerated actually causes the severing of the detonating cord. For the '249 patent, the severing means is a metal foil sleeve placed over the detonating cord at an actual crossover point.
Both of these packaging methods require that the detonating cord be bent back to either cross over itself or pass dose by in a parallel orientation to insure severing of the detonating cord. Placing severe bends in the detonating cord is detrimental if the cord remains in this packaging orientation for an extended period of time.
In the preferred embodiment of this invention, the detonating cord can be space apart at a greater distance that allows the radius of the loop to be increased to avoid damaging the detonating cord. Also the cord sections pass each other at the severing location in an arcuate configuration that avoids any sharp bends in the detonating cord. This packaging method allows the detonating cord to be stored in this configuration for an extended length of time. Also since no separate severing means is required, there will be a resultant material and labor costs.
It is the objective of the present invention to provide an improved method for packaging detonating cord that will meet the requirements for shipment by commercial air carriers in the United States and internationally. It is another objective of the present invention to obviate the need for a severing means and instead rely on the detonation properties of the cord to sever itself. It is another object of the present invention to provide an explosive diode whereby the propagation of detonation is restricted to one direction only.